This invention is an instructional aid for pilots learning the techniques of instrument flying. Specifically, this device is useful in learning the techniques for navigating by reference to nondirectional beacon (NDB) radio transmitters and making NDB instrument landing approaches (blind landings).
In aviation, atmospheric conditions are classified as "visual" or "instrument." Visual conditions prevail when the ground is visible and the pilot can see for a considerable distance in front of the aircraft. Instrument conditions prevail when the visibility is poor enough to require navigation solely by cockpit instruments.
To fly under instrument conditions, a pilot must acquire an "instrument rating" in addition to his pilot's license. An instrument-rated pilot may (within certain limitations) fly through clouds and rain, navigating by reference to special radio transmitters on the ground. Upon reaching his airport of destination, the pilot must execute an instrument landing approach. This carefully planned procedure (published on government maps) will guide the pilot from an altitude of several thousand feet down to a few hundred feet above the ground, and will position him over the end of the runway headed in the proper direction for landing. The instrument landing approach is the most precise and critical part of an instrument flight.
There are three types of instrument landing approaches, classified according to the type of radio transmitters used: they are the VOR, ILS, and NDB approaches. This invention relates to training pilots in proper utilization of the NDB transmitter for both in-flight navigation and making instrument landing approaches.
The NDB is the oldest of the three types of approaches, it is the least precise and the most difficult for the pilot to execute. It is still in wide use, however, because it utilizes a very simple and inexpensive transmitter. Hundreds of small airports have only NDB approaches; many large airports have an NDB approach as back-up, to be used if the VOR or ILS transmitter fails. The Federal Aviation Administration requires each pilot applying for an instrument rating to demonstrate competence in NDB approaches.
The NDB approach system consists of a ground radio transmitter (located on or near the airport), a special radio receiver in the aircraft, and a detailed map of the approach procedure.
The ground transmitter is the nondirectional beacon which gives the approach its name. It is a simple low frequency (200 to 415 kilohertz) transmitter which broadcasts a signal which is uniform in all directions (hence nondirectional). The carrier is modulated with an audible tone; the tone is interrupted to create a Morse code identification. Each NDB has a different three-letter identification so that the pilot may be sure he has tuned in the proper transmitter.
The airborne receiver is an automatic direction finder which tells the pilot which direction the radio signal is coming from. This information is displayed in the cockpit on a relative bearing indicator. The relative bearing indicator is a circular instrument calibrated with an azimuth of 360 degrees. In the center of the circle is a rotating needle with an arrow on one end. The needle indicates the direction (bearing) of the NDB transmitter relative to the airplane. If the needle comes to rest at 0.degree., the transmitter is directly in front of the airplane. If the needle points to 90.degree., the transmitter is off the right wingtip; at 180.degree. it is behind the airplane; at 270.degree. it is off the left wingtip. Whenever the airplane is within range of the NDB transmitter, the pilot may readily observe the direction of the transmitter relative to the airplane.
The relative bearing indicator tells the pilot the direction of the NDB in relation to the airplane, but it does not tell him the direction of the airplane in relation to the NDB. If, for example, the needle of the relative bearing indicator points to 0.degree., the pilot knows that the NDB is directly ahead. But he does not know whether he is north, south, east, or west of the NDB. To learn his direction from the NDB, the pilot must consult the compass. (The pilot would typically use the directional gyroscope or "gyro compass" instead of the harder-to-read magnetic compass.) If the compass shows that he is heading 90.degree. or due east, while the relative bearing indicator shows 0.degree., then the pilot concludes that he is due west of the NDB.
Throughout an NDB landing approach the pilot must read the relative bearing indicator and the gyro compass, perform some mental arithmetic to determine his direction from the NDB, estimate the winds, add a correction for wind drift, then repeat the process.
NDB navigation is a complex process which requires a clear understanding of geometric relationships, the ability to visualize and anticipate changing angles, and the rapid mental manipulation of figures. It is very difficult to learn NDB navigation while flying an airplane, and training is best conducted on the ground until the student has mastered the necessary mental skills. The invention disclosed here is a simple, inexpensive device which can greatly facilitiate the learning of these skills.